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8 MAINTENANCE
8.1 Chemistry
Neither ammonium ion content nor total ammonia can be
measured directly in an untreated sample, since the probe can
respond only to free ammonia gas. The ammonium ions must
therefore be converted to free ammonia by adjusting the pH of
the sample solution to a value greater than 11. This is effected
by addition of a sodium hydroxide solution to the sample
before it is presented to the probe. This reagent also has a
second function, i.e., to fix the total concentration of dissolved
species in the final solution at approximately 0.2M.
In water industry applications a water softener, EDTA, is
added prior to the sodium hydroxide addition. Besides
preventing hardness precipitation, this also overcomes
inaccuracies caused by complexing of ammonia with some
metal ions. In power applications where the hardness levels
are very low this second reagent is not required, so the bottle
tube, pump tube and first reaction coil can be removed.
The reagent and standard solutions described below are
required to maintain the monitor in operation. Where possible,
they should be freshly made and stored in plastic (e.g.
polythene) bottles. Typical reagent solution consumption for a
continuously operating monitor is approximately 10 litres per
month. The monitor uses 50 to 80 ml of each standard solution
each calibration cycle; consumption of the standard solution,
also depends on the frequency with which this cycle is carried
out.
8.1.1 Reagent Solutions
Two reagent solutions are used in the ammonia monitor. Care
must be exercised to ensure that the correct reagent is fed into
each channel. They are prepared as follows:
Reagent 1 – Orange Channel
Dissolve 315 (±5) g laboratory reagent grade di-sodium EDTA
in approximately 8 litres of high purity water and make up to 10
litres with more high purity water.
Note. If the monitor is to be used on power plant
applications the reagent will not be required due to the
insignificant concentration of hardness in the sample.
Reagent 2 – Red Channel
Dissolve 1000 (±10) g analytical reagent grade sodium
hydroxide (NaOH) in approximately 8 litres of high purity
water. Allow the solution to cool and then make up to ten litres
with more high purity water.
Warning. This solution is extremely caustic; avoid
contact with the skin and eyes. If contact occurs, wash
with plenty of clean water.
8.1.2 Standard Solutions
Two standard solutions of known concentration appropriate to
the measuring range are required for calibration of the monitor.
To prepare a stock solution of 10,000 mg l
below:
a) Ammonia as N – Dissolve 38.21 (±0.01) g analytical
reagent grade ammonium chloride (NH
approximately 200 ml of high purity water.
Ammonia as NH
– Dissolve 31.47 (±0.01) g analytical
3
reagent grade ammonium chloride (NH
approximately 200 ml of high purity water.
Ammonia as NH
+
– Dissolve 29.72 (±0.01) g analytical
4
reagent grade ammonium chloride (NH
approximately 200 ml of high purity water.
b) Transfer the solution to a one litre volumetric flask and
make up to the mark with more high purity water.
c) Dilute the appropriate stock solution with more high purity
water to make the two standard solutions for the measuring
range of the monitor. Store in plastic bottles.
Note. Low value ammonia standard solutions should be
avoided. For example, at ambient temperatures of about
25 °C, and under normal light conditions, a 0.5 mg
solution is expected to lose at least 10% of its
concentration over a seven day period. Because of the
instability of such solutions, it is not possible to
programme a standard solution of lower concentration
than 0.2 mg l
–1
NH
on this monitor. Solutions as low as
3
–1
0.2 mg l
can be prepared with care, but must be used
immediately, making it an unsuitable concentration for
long term use with the monitor.
Note. The mass relationship of ammonium (NH
ammonia (NH
) is 18/17 and ammonia (NH
3
(N) is 17/14.
8.2 Scheduled Servicing
The following servicing schedule has been produced as a
general guide only. Because the monitor has been designed
for a very wide range of applications, where the nature of the
sample can vary considerably, it may be necessary to amend
the schedule to suit the particular installation and sample
conditions.
8.2.1 Regular Visual Checks
It is recommended that the monitor and the sampling system is
visually inspected on a regular basis to assure correct
functioning of the system and to check the integrity of the
readings.
a) Check for leaks, particularly around the sample and drain
pipework connections.
b) Confirm sample flow by checking delivery to the constant
head unit and effluent from the drain.
c) Check liquid flow over the probe membrane.
–1
follow instructions
Cl) (dried) in
4
Cl) (dried) in
4
Cl) (dried) in
4
-1
NH
3
+
) to
4
) to nitrogen
3
29
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